Feb. 4, 2009 -- More than 2 million young children require medical treatment for respiratory syncytial virus every year, yet most never receive a confirmed diagnosis, new research shows.

Respiratory syncytial virus (RSV) can cause infection in people of all ages, but young infants and children are especially at risk for severe disease. Nearly all children will have experienced an RSV infection by age 2. Illness severity can range from a cold to more serious respiratory infection, such as pneumonia and bronchiolitis (inflammation of the small airways).

Outbreaks typically occur in the winter months. RSV infection is the leading cause of wintertime hospital admissions among babies.

In a study to examine the burden of RSV in all children younger than age 5, researchers concluded that the virus is responsible for as many sick-child visits to doctors each year as influenza and causes three times as many hospitalizations.

Most RSV Infections Aren't Diagnosed

The study, which appears in the Feb. 5 issue of the New England Journal of Medicine, included 5,067 children under age 5 who were hospitalized or treated in emergency departments or doctors' offices for respiratory infections during several winter RSV seasons.

Based on their findings, the researchers estimate that RSV infection is the cause of one in every 13 outpatient visits to physicians each year by children under age 5, and one in 38 emergency department visits.

Researchers also concluded that:

The majority (78%) of children with RSV are older than 12 months, and most have no other medical conditions that would place them at high risk.

Only 3% of children with the virus who are not hospitalized received a diagnosis of RSV for their illness.

Lead researcher Caroline Breese Hall, MD, of the University of Rochester Medical Center, tells WebMD that many pediatricians do not test for RSV because there are no specific treatments for the virus.

Many also end up prescribing therapies that do no good, such as antibiotics, because RSV symptoms closely mimic those of respiratory diseases caused by bacterial infections.

"We in the health community need to recognize that the burden of RSV infection is larger than we have realized, and we need to look for ways to protect children even after their first birthdays," she says.

RSV Vaccine Needed

Preventive treatments are available, but they are used primarily in high-risk infants and young children because they are very expensive.

Early attempts to develop an RSV vaccine that could be given to infants were unsuccessful. A vaccine tested in the 1960s ended up making some infants sick.

But Hill says the same vaccines that did not work in infants may be effective in children over 12 months because their immune systems are more developed.